This invention relates to a magnetic field generator for use with an insertion device in order to produce radiations having various polarization characteristics, as well as a method for generating magnetic fields and a method of producing polarized radiation.
It is well known that when high-energy electrons accelerated by a particle accelerator such as a synchrotron are subjected to motion in a periodic magnetic field, radiation of high directivity and very high luminance are produced over a spectral range from the ultra-violet to X-ray region. In particular, undulator radiation is very useful since it is 2-4 times more intense in magnitude than the light emitted from bending magnets and is quasimonochromatic. Such radiation is produced by means of a special light source called an "insertion device".
Conventional insertion devices consist merely of two sets of magnet arrays, each set being provided above and below the plane of an electron orbit in order to generate sinusoidal periodic magnetic fields, thereby producing a horizontally polarized radiation, or radiation polarized linearly in a horizontal plane. In certain applications, increasing use is made of either vertically polarized radiation, or radiation polarized linearly in a plane perpendicular to the plane of an electron orbit (vertical plane), or circularly polarized radiation. Consider, for example, fields such as structural phase transfer, diffuse scattering and biopolymers, the vertically polarized light is used in these applications whereas the circularly polarized light is used in other fields such as magnetic scattering and solid electron spectrometry. Kwang J. Kim, Nucl. Inst. Meth, Phys. Res. 219(1984) 425-429 reported an insertion device in which, two sets of magnet arrays are provided, one set being horizontal magnet arrays and the other being vertical arrays, so that two sinusoidal periodic magnetic fields are crossed at right angles on the axis of an electron orbit to produce elliptically or circularly polarized radiation.
It is theoretically impossible to produce circularly polarized radiation with the first type of insertion device. On the other hand, it has been impossible for the second type of insertion device to pick up radiation at a wavelength as short as those obtainable from the first type. This is because the period length of periodic magnetic fields must be increased in order to attain a sufficient field strength on electron orbits to withstand practical applications.
The second type of insertion device permits the gap in the horizontal direction to be made as small as the gap in the vertical direction and, hence, it is theoretically possible to produce satisfactory magnetic fields on electron orbits at short wavelengths. However, the second type of insertion device is limited in its ability to generate an even stronger magnetic field on electron orbits by reducing the distance between the magnet arrays on the right and left sides of an electron orbit. This is because the aperture for electron beams in the horizontal plane is limited by those two magnet arrays. A further problem with the second type of insertion device is that no satisfactory degree of circular polarization can be achieved if electron beams are divergent (accelerated electron beams are divergent in all cases).